Optically excited lasers, the amplifying medium of which comprises a crystalline or glass-like solid, which is also called host crystal or host material, are generally referred to as solid-state lasers. The host crystal is doped with ions of an extraneous material. By the introduction of energy into the laser medium electrons are energized to a higher energy level so that excited atoms result. This process is also referred to as pumping. Normally, solid-state lasers are optically pumped. When an atom which has been excited such is stimulated by a photon having the energy to be emitted, then the excited atom again falls back into its normal state and thereby emits a photon with identical energy as well as identical phase position like the stimulating photon. The directions of motion of both photons are the same. Due to this duplication of the stimulating photon the laser medium acts as a light amplifier which is also referred to as classical “lasing”. In the prior art, such solid-state lasers are well-known. Mostly, laser activity is initiated by spontaneous emission through which electrons statistically, after a material-specific half-life, fall back into their normal state and thereby emit a photon having an individual direction and phase position each. A laser operation is typically reached, when in addition to the active medium a cavity of at least partially reflecting mirrors exists, between which photons having a certain direction are again guided through the active medium and then stimulate further emissions. Then, photons having this direction and having the same phase will prevail. Normally, photons which are emitted in another direction by spontaneous emission will not be amplified.
A well-established host crystal is sapphire. Sapphire is a crystalline form of Al2O3 which is characterized by a particularly high hardness. A sapphire crystal is an optically negative uniaxial crystal which can be described by the crystallographic axis c and the crystallographic axes a and b being perpendicular with respect to c and with respect to each other. In the case of sapphire, the crystallographic axis c is the optical axis. In the case of sapphire, the highest amplification of light can be achieved for radiation which propagates orthogonally with respect to the optical axis. But since both, the axis a and also the axis b are orthogonal to the axis c, there are thus two axis (a, b) which are again respectively orthogonal to each other, along which amplification of light may arise. However, normally, only an amplification in one of both directions is desired so that only in one of these directions an exit of the amplified light is envisaged. The amplifications of spontaneous emissions arising perpendicularly with respect to this preferred direction are also referred to as parasitic laser activities. By the parasitic laser activities further excited atoms are stimulated to emission without the possibility of using the emitted photons for the desired laser operation, because they do not propagate along the preferred orientation, but in a perpendicular orientation thereto. Thus, the parasitic laser activities relate to a reduction of the efficiency of the laser, because energy being pumped into the laser medium remains unused, thus is lost in this way. So, the efficiency of the laser will be reduced.
Spontaneous emission in directions which are different from the laser direction cannot be suppressed within the laser medium. However, this phenomenon is particularly problematic, when at the boundary surface between the sapphire crystal and the surrounding medium (normally air) a proportion of the light is reflected back into the crystal or is reflected back and forth even several times and thus provides a further amplification of the parasitic laser activity and dissipates further energy by means of stimulated emission. This case is referred to as parasitic oscillations. When the amplification (also gain) of laser light in the active medium in a parasitic direction is high, then only low reflectivity is enough, such as e.g. in the case of a not non-reflecting boundary surface between the laser medium and air, for causing a considerable loss of energy. Thus, the parasitic laser activity can be attenuated by preventing the back reflection of the light being perpendicular to the direction of the radiation of the laser into the sapphire crystal or by at least attenuating it considerably.
In the case of doped sapphire the amplification of the light originating from parasitic laser activity (parasitic laser light) is so high, in particularly orthogonal to the preferred amplification orientation and orthogonal to the crystallographic axis c of the sapphire, that even also in the case of an antireflection coating on the lateral surface of the laser crystal strong reflections occur so that this problem is solved by the use of measures which suppress the reflections of parasitic laser light still further.
Such a solution is the following: bringing diiodine methane (as a liquid cladding material) in contact with the lateral surface of the laser medium. This liquid has an adjusted refractive index and absorbs the parasitic light so that reflections are suppressed. But this liquid has several decisive disadvantages. Firstly, the fact that it is a liquid and not a solid results in a more complicated and failure-prone laser system. Secondly, the area via which the laser medium is in contact with the liquid is limited, since the laser medium has to be held and fixed also beyond the laser areas. Thirdly, this liquid is hazardous to health. Fourthly, this liquid decomposes over time and has to be replaced at regular intervals. Fifthly, for such a solution of the problem the refractive index has to be adjusted over a certain wavelength range and, while the reflective index at the central wavelength can minimally be changed by means of addition of additives, the dispersion (the dependency of the refractive index on the wavelength) is constant more or less and is different from that of the sapphire.
Therefore, there were some attempts to reduce the parasitic oscillations with a solid cladding and thus to increase the efficiency of the laser. One principle of function which is based on the suppression of parasitic light by the use of a cladding glass is described in U.S. Pat. No. 4,217,382 A, but this invention relates to the special case of neodymium-doped fluorophosphate glass. In this system, the wavelength of the parasitic laser light to be suppressed is 1052 nm and the refractive index of the active medium is ca. 1.46, i.e. the invention which is described in U.S. Pat. No. 4,217,382 A is not at all suitable as a cladding for doped sapphire. Therefore, it is an object of the present invention to provide a means which in the case of doped sapphire is suitable for suppressing parasitic light in the visible range and overcomes the disadvantages of the glass known from prior art. The object is solved by the subject matter of the patent claims.